1. Field of the Invention
The present invention relates to a delay-time-modulation effecting apparatus which effects a delay-time modulation on musical tone signals produced by an electronic musical instrument and the like.
2. Prior Art
In the electronic musical instruments recently developed, a variety of musical-tone parameters are controlled in real time. One method for controlling the musical-tone parameters utilizes manual-operable members such as a wheel-type controller and a pedal which are equipped in the electronic musical instrument. Another method utilizes information, given from an external device, such as the information regarding "control change" of the MIDI system (where `MIDI` is acronym for Musical Instrument Digital Interface). A still another method utilizes an output signal of a low-frequency oscillator (i.e., LFO). Moreover, another technology is recently developed in such a manner that plural pieces of information, which are used to control the musical-tone parameters, are combined together to form complicated modulation information, by which the musical-tone parameters are controlled so that rich expression in the musical performance can be obtained. Such technology is disclosed by the papers of Japanese Patent Laid-Open No. 3-126090 and Japanese Patent Laid-Open No. 4-138499.
By using an effect circuit, which is disclosed by the papers of Japanese Patent Laid-Open No. 58-108583, in the delay-time-modulation effecting apparatus, the delay-time modulation can be performed so that the chorus effect or symphonic effect can be realized. In some cases, however, when using the modulation information which is controlled by a plurality of control members, there is a possibility that the read address advances fast to be ahead of the write address. When the read address advances ahead of the write address, a discontinuity should be occurred in read signals, which may initiate the generation of the noises.
In order to avoid the generation of the noises, the conventional apparatus utilizes offset values or coefficients of modulation which are determined in advance under the consideration of the maximum value (i.e., maximum amplitude) set for modulation signals.
However, if the offset values are determined in response to the maximum amplitude as described above, the amount of delay in the normal frequency range should become large. This causes a relatively large shift in phase between the original sound and the sound artificially produced. In addition, if the coefficients of modulation are determined in response to the maximum amplitude, there is a possibility that the amplitude of the modulation signal in the normal frequency range should be smaller than an instructed value. Thus, in order to limit the maximum amplitude, a range in the variation of the values set by the controller, which controls the musical-tone parameter in real time, should be limited; or a degree of modulation should be limited. However, there is no conventional apparatus which satisfies the above demands.